1. Field of the Invention
The present invention relates to voice coil motors. More particularly, it relates to a voice coil motor for driving a movable magnetic head in a magnetic disk storage device, and an apparatus for measuring the center of the thrust of the voice coil motor.
2. Description of the Related Art
Heretofore, linear actuators have been extensively used. A typical example of a linear actuator is a voice coil motor.
The voice coil motor is operated on the same principle as a loudspeaker. It is mainly constructed of a voice coil, and a magnetic circuit which has a gap for inserting the voice coil therein and by which a magnetic flux traversing the voice coil is generated in the gap. In addition, the magnetic circuit is mainly configured of a permanent magnet and a yoke. With the voice coil motor, a thrust acting in the axial direction of the voice coil is developed in such a way that current flowing through the voice coil is changed according to the state in which the magnetic flux traverses this voice coil.
The voice coil motor plays an important role in the case where a magnetic head, a pickup or the like is positioned on a recording medium at high speed and with high accuracy in a magnetic disk storage device, an optical disk storage device, or the like.
The positioning is effected in such a way that a carriage on which the magnetic head, the pickup or the like is mounted is moved relative to the recording medium by the voice coil motor. Accordingly, the position of the center of gravity of the carriage and the center of the thrust developed by the voice coil motor needs to lie on an identical axis with the moving direction of the carriage. In a case where they do not lie on the identical axis, part of the thrust turns into a couple of forces, which incite the carriage. As a result, the carriage is vibrated to degrade the positioning accuracy of the magnetic head, the pickup or the like.
Therefore, various countermeasures as stated below have hitherto been taken.
In the official gazette of Japanese Patent Application Laid-open No. 211363/1983, there has been proposed a structure wherein a pair of voice coil motors are disposed at positions between which the carriage to be guided on a rail is centrally held.
Besides, the center of the resultant of the thrusts of the pair of voice coil motors is brought into agreement with the position of the center of gravity of the carriage. Thus, the couple which is one cause for the occurrence of the vibrations is prevented from acting on the carriage.
In addition, the official gazette of Japanese Patent Application Laid-open No. 10379/1988 has proposed a technique wherein the actuator is driven in the same manner as in the actual use of the voice coil motor and wherein the position of the center of gravity of the carriage or that of the center of the thrust of the voice coil motor is regulated so as to minimize vibrations occurring during the drive. As a practicable method, it proposed to form a hole in part of the yoke and to insert a rod of ferromagnetic substance into the hole, whereby the distribution of magnetic flux densities in the gap of the voice coil motor is adjusted to regulate the center of the thrust.
Further, the official gazette of Japanese Patent Application Laid-open No. 177854/1989 has proposed a technique wherein a chip of magnetic substance for adjusting the distribution of magnetic flux densities in the gap is stuck to part of the gap, thereby regulating the center of the driving force.